chore(Gruntfile): adds grunt-contrib-watch support

this commit introduces a new grunt-contrib-watch plugin to
let a watcher run in the background when developing on
angular-leap. Currently, it just executes the jshint
task that was introduced a commit before this commit.

I'll expand the watch task by adding karma tasks as well,
to run all provided tests when a change on any source file
occurs. But thats up next :)
chore(Gruntfile): adds grunt-karma support

adds grunt-karma plugin as depedency and makes use of it
as a watch task anytime a source file changes.

this means, when now running a grunt watch for development
purposes, everytime a source file changes, not only jshint
is run to make sure you don't have any syntax errors n stuff,
but also a karma single run is executed, to make sure all
tests are always green :).
Merge pull request #8 from angular-leap/grunt-karma-support

chore(Gruntfile): adds grunt-karma support